An integrated circuit comprises a large number of semiconductor devices, such as transistors, that are formed on a semiconductor substrate or, more colloquially, a “chip.” These devices are selectively interconnected by one or more patterned layers of a conductive material, typically aluminum, to form a circuit that performs a desired function. These layers are referred to as “metallization” layers. As integrated circuits become more complex, designers reduce the minimum feature size of the constituent devices of the circuit, so as to fit more devices on a chip. With this reduction in size, it becomes more difficult to achieve proper pattern definition using conventional techniques such as photolithography and dry etch techniques for forming metallization layers. Further, designers have attempted to use copper instead of aluminum as the principle metallization material in the metallization layers, due to perceived advantages in resistivity, ductility and melting point. Unfortunately, developers have not been able to create a reliable technique for patterning a copper layer.
For example, one process using electro-deposition for forming a copper metallization layer is described in U.S. Pat. No. 5,151,168. According to this process, a conductive barrier layer is deposited on a semiconductor substrate. Further, a photoresist reverse image of the maskwork normally used to etch the metallization pattern is created on the substrate. The wafer is then transferred to an electrolytic bath in which the copper is complexed with EDTA molecules. A fixed voltage is applied between a voltage source and the semiconductor substrate to deposit the copper ions on the barrier layer that is not covered by the photoresist layer including contact/via openings on the semiconductor substrate. Unfortunately, when the substrate is placed in the electrolytic bath, the photoresist material may lift-off from the substrate thus depositing copper in areas where it is not required.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a method for forming a metallization layer that avoids the disadvantages and problems of prior techniques.